Devices by which a person visiting a home may generate a ring-tone in the home, such as a doorbell, a video phone, a door phone, and the like, have been generally used. For instance, when the visitor pushes the button of a door phone, a video of the visitor, as taken by a camera, can be displayed on a smart terminal in the home. Then, a user in the home can verify the video of the visitor and make a call (depending on a call connection between the smart terminal and the door phone) to the door phone, or push a button to unlock a door lock, thereby making it possible to open the door remotely.
However, in a situation in which it is difficult to respond through the smart terminal, e.g., when there is nobody in the home, an unspecified visitor can be informed that there is no one home, which may pose safety concerns. Similarly, in a situation where only elderly or sick persons are in the home, those persons may feel uneasy due to a visit of an unspecified visitor.
In order to solve this problem, technology has been developed where a call can be made to a mobile terminal, such as a smart phone, or the like, of a customer (who is registered in advance) when the button of the door phone is pushed. However, even in this case, a safety concerns still exist. For instance, a driver of a vehicle may be distracted due to a sudden call from a visitor while driving.